Schlieren photography

Hot air rises and cold air falls—but how can we be certain? Jet
planes screaming overhead leave shock waves in their wake—but,
again, how do we really know? One way we know is from taking
Schlieren photographs, which show up invisible changes in the air
as streaky patterns of color or shades of gray. (The word "Schlieren" is pronounced
sh-leer-un
and comes from the German word for streaks.)
Images like this have been widely used by airplane
designers and space scientists for decades, though the basic
technique is simple enough for almost anyone (with the right
equipment) to try at home. Let's take a closer look at how Schlieren
photography actually works!

Photo: How does a plane's nose cut through the air? This colored Schlieren photo, taken in a wind tunnel, gives you some idea. The plane is the dark shape at the bottom, pointing up. The lines you can see are shock waves created as its nose cuts through the air. Photo by courtesy of NASA Ames Research Center (NASA-ARC).

What is Schlieren photography?

Photo: What difference does the shape of a plane's wings make to the shock waves it creates at supersonic speeds? Schlieren photos can tell you! Photo by courtesy of NASA Ames Research Center (NASA-ARC).

You often see Schlieren photos in science books, but they're not
widely used in newspapers, magazines, or other mass media, perhaps
because they take a little bit of explaining. Even so, the basic idea
is fairly easy to understand.

Suppose you place a hot cup of coffee on your desk. If it's really
hot and the lighting's just right, you might be able to see steam
swirling up from the cup and disappearing into the air above. What
you won't be able to see is the full effect of the convection
currents: how hot air rises above the cup while cool air moves in
underneath to take its place. Why does this happen? Hot air is less
dense than cooler air and so, effectively, lighter. You can almost
think of it as a second, lighter gas. Because it's lighter, it's less
affected by gravity and that's why it rises above cooler, heavier
air. (Read more about this in our article on heat.)

How, then, can you make convection currents visible? Suppose you
shine a lamp through the air where your coffee cup is standing. You
probably know that light travels faster in substances that are less
dense and slows down a little when it moves into denser substances.
(That's why a straw or a spoon appears bent when it stands in a glass
of water: light travels quicker in air than in water.) So light rays
traveling through our hot coffee steam will travel a bit faster (and
be bent or refracted less) than light rays traveling through cooler air nearby
(which will travel slower and be bent more). With the help of a good
light source and a mirror, we can make the fast light rays follow a
slightly different path from the slow ones. If we stick a colored
filter in their way, along with a camera, we can color the fast rays
(say, red) before they enter the camera lens and make the slower rays
a different color (say, green). What we end up with is a colored
photo that reveals the swirling patterns of the air caused by the
heat from our coffee: a Schlieren photo!

How do you take Schlieren photographs?

Photo: Art or science? The different colors in a Schlieren photo are caused by light traveling at different speeds through different parts of the airstream, so being bent to travel through different parts of a colored filter. This 1971 Schlieren photo was taken as part of early design tests of the Space Shuttle. Photo by courtesy of NASA Ames Research Center (NASA-ARC).

There are lots of websites that describe the practical details
very clearly and I'm not going to repeat them here (if you're
interested, simply type "how to take Schlieren photography" into your favorite search engine). But
here's the basic setup. You aim a collimated, point source of
light (essentially, filtered and focused light) at a large, spherical, optical-quality
mirror (your grannie's old mirror
simply won't do—the surface will be too irregular and it will blur
the light rays too much). You place your camera next to the light and
put either a sharp knife blade or a filter with bands of color in
front of it, so it partly obscures the lens. Then you simply place
the object you want to photograph (say, your steaming hot cup of
coffee) in between the light and the mirror.

How does the photograph form? The filtered light passes twice
through the hazy airstream near your coffee. Light rays traveling
through cooler, heavier air are bent more than rays through hotter,
lighter air so, after being reflected by the mirror, rays that have
traveled through different parts of the airstream are shifted
slightly to one side or the other. If you're using a colored filter,
some rays pass through one of the colored areas while other rays pass
through a different colored area. That's how the photograph your
camera captures shows up differences in the air density as
streaks of color. If you're using a knife blade instead of a
colored filter, you get a black-and-white Schlieren photo with the
density differences shown up as streaks of gray.

One thing to note about Schlieren photos is that they give only a
rough, visual impression of air density changes rather than the
precise measurements you might make with computer models or
wind-tunnel tests. In other words, this is a qualitative
rather than a quantitative technique.

Large-scale Schlieren photography

How do you take photos of really big things, like the airstream round a space rocket? Here's a method that NASA developed in the 1970s,
which uses a pair of gratings either side of the fluid being photographed instead of the usual knife blade.
Following the numbering scheme they've used, and reading from right to left:

12 is a point source of light.

16 (red) is a condensing lens system.

14 (gray) is a small grating (roughly 5cm by 5cm with 400 lines per cm, or 2in by 2in and 1000 lines per inch).

20 (red) is a diverging projector-type lens.

22 (blue) is the fluid object being photographed (such as a disturbed airstream).

18 (gray) is a large grating made from steel rods (roughly 1/25cm or 0.5 inch in diameter and spaced 15cm or 40inches apart) with thin black nylon twine stretched between them.

24 (green) is a viewing screen on which an image of the object is projected.

26 (purple) is an ordinary camera for photographing the image on the screen.

How does it work?

Light from the source is concentrated and focused by the condensing lenses, passes through the grating, which breaks it into multiple pieces, and then projected through the fluid object by the diverging lens on the other side. The fluid object refracts and shifts it slightly, forming a Schlieren image on the screen, which can be photographed in the normal way.

What else can Schlieren photographs be used for?

Most Schlieren photos are used for studying the air movements
caused by fast-moving objects, such as
airplanes, bullets,
space rockets, and so on. Generally, they're taken in wind tunnels, though
it's also possible to take Schlieren photos of real planes in
flight using the Sun as the light source (like the one shown here). You can also take
simulated versions of Schlieren photos using computer programs that
mimic the process. This technique is usually called synthetic
Schlieren photography.

Photo: It's possible to take Schlieren photos of real planes. This one was taken by
Dr. Leonard Weinstein of NASA Langley Research Center in December 1993 and shows a T-38 flying at
an altitude of 4,000m (13,000ft), at a speed of Mach 1.1, over Wallops Island, MD. Photo by courtesy of NASA Armstrong Flight Research Center.

Who invented Schlieren photographs?

It wasn't a person called Schlieren; as we saw earlier, that German word ("streaks") describes what the photos look like. The technique was actually developed by German physicist
August Toepler between the late 1850s and
the mid 1860s.

Schlieren and Shadowgraph Techniques by Gary Settles. Springer, 2001. A practical guide, including the history of Schlieren photography, from one of the world's leading Schlieren practitioners
(Distinguished Professor of Mechanical Engineering at the Pennsylvania State University).

US Patent 9,232,117: Digital Schlieren imaging by Benjamin Buckner et al, Metrolaser, January 5, 2016. A modern, digital method of using a type of Schlieren photography known as complementary-grid focusing for taking much larger photos.